Crude unit furnace tubes

After the reactions have occurred in the catalytic crackers, cokers, and alkylation plants, the effluents are separated by fractionation to obtain the various products desired. The materials for the fractionation equipment are selected using basically the same criteria as those used for crude units, as discussed in Chapter One [i.e., carbon steel below 550 to 600°F (288 to 315°C) and 5Cr-V6Mo piping and 12Cr clad vessels above]. However, there are some slight differences from crude units. An example is furnace tubes in delayed cokers, which are usually 9Cr-t Mo (because of the high-fluid temperatures) rather than 5Cr-V Mo. 

Fig. 8 is a schematic representation of a vacuum column, sometimes called a flasher, intended to produce asphaltene-free cracking feed and heavy black fuel oil. The fired preheat furnace is similar to the crude unit heater, except that tolerable tube skin temperatures are somewhat lower because of the absence of light hydrocarbons. 

Principal uses of anti-foulants are in hydrodesulfurizers (for naphtha, gas, and lubricating oUs), in naphtha reformers, in crude and catalytic cracking units. Other units include cokers, visbreakers, alkylation units, ethylene units, deethanizers, solvent recovery units, etc. While fouled equipment consists primarily of heat exchangers, furnace tubes, piping, and distillation towers can also be affected. 

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